Question

as引导的定语从句位于句首时做的不是宾语

Answer 1

as 引导的定语从句位于句首时，as 是关系代词，它可能做宾语，也可能做主语。例如：
1. As we all know, the earth goes round the sun. 我们都知道，地球环绕太阳转。此句中 as 做宾语。
2. As is known to all, the earth goes round the sun. 众所周知，地球环绕太阳转。此句中 as 做主语。
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Answer 2

as 引导定语从句时，其先行词是整个句子的内容，as在定语从句中的成分可以是主语也可以是宾语，不管它引导的从句是在句首还是句末。
As is known to all,the matter has three states. as在从句中 充当主语
As we all know ,the matter has three states. as在从句中 充当 宾语

Answer 3

as is known to all, she is the best player in the team. 在这句话里，as是做定语从句的主语的，指代she is the best player in the team这件事情是众所周知的。